How to Upgrade a C3 Corvette to LED’s: 1972 Corvette Scarlett Project Car

We’ve spent the last several months wiring Scarlett, our ’72 coupe project car, adding fuses, relays, electric windows and other features to make the car more user-friendly once she’s back on the road. For our encore, we’ve saved the part of the electrical system probably most used, and almost certainly the most important: lights. It’s no secret Sharks came with lousy lights; the 5-3/4-inch round, sealed beams may have been cutting edge in the late ’60s, but they’ve long since been supplanted by superior technology.

And when we say “lights,” we don’t mean just the headlights, although that’s our main focus. We replaced every bulb, from the corner marker lights to the gauges, with LEDs. While it’s not correct to say they never wear out, LEDs have a much longer life than traditional filament bulbs—and use less power—placing less load on the electrical system. The only negative to the reduced current draw of LEDs is the effect it has on the factory flasher units, which depend upon thermal load in order to work.

In short, electricity builds up heat inside the thermal flasher, causing it to expand and contract, making and breaking contact, which creates the “flashing” effect of your turn signals. If there’s not enough electricity (and there generally isn’t from the low draw of an LED), the flasher won’t cycle correctly. There are two ways to fix the problem: add a load resistor, which obstructs the power flow into (and out of) the LED, or use a specialized LED flasher unit that doesn’t depend on heat buildup to work. Since the load resistor reduces the benefit in current reduction offered by the LED, the LED flasher unit seemed preferable, so we ordered the three flashers from Corvette America and installed those when we ran the dash harness. It was as simple as plugging them in and connecting the included ground wire.

While there are many sources for retrofit bulbs, there are fairly strict rules that govern things like output at a variety of angles and effective lit area, and the ones recently introduced by Philips under its Vision LED line are some of the very few legitimate retrofit LEDs. Although several companies offer a complete taillight unit that uses a cluster of LEDs contained in a single, factory-like housing, we chose the Philips option because it was simpler and let us keep the factory lens and appearance. We ordered two pair for the tail and brake lights, and installing them was as simple as unscrewing the red plastic lenses, unplugging the bayonet-style 1156 backup and 1157 stop bulbs, and replacing them with the Philips units. Since white LED’s have a tendency to “wash out” the color of the plastic lens they’re behind, we used red bulbs for the taillights.

The headlights required a bit more to install. We reached out to lighting consultant Daniel Stern to explore available options and selected an H1 halogen conversion using a Hella Bifocal housing for the low beam and a similar Cibie parabolic housing for the high beam. While there are H1 HID conversions available, the optics aren’t optimized for a 5-3/4-inch housing, so we kept it simple. While the rear of the Cibié housing fit without modification, the larger Hella housings required some surgery on our headlight buckets. The wiring, however, was simple.

Starting with the factory headlight plugs on one side of the car (we picked the passenger side, since it’s closer to our power), the components we ordered from Stern let us build a subharness that plugs into the factory plug without cutting any wires. Basically, for both high and low beams, the female factory plug provides the ground and trigger wire that operate a relay that delivers the power, via 12-gauge wires, from the relay to the lights, keeping the heavy amperage required to run the lights out of the passenger compartment, where it usually runs through the light switch itself. We powered each relay with a 10-gauge feed from a fused distribution block and added a common ground stud to the rear of the headlight bucket assembly so all the extra ground wires from the bulbs didn’t have far to go.

Instead of the skirted Hella relays we’ve been using, Stern recommends German Flosser relays that have dual outputs, which let us wire both high beam feeds from one of the two factory high beam plugs, and both low beams from a single low beam plug. The only modification to the factory harness (which should be easy if you stocked up on Packard 56 terminals) is adding a jumper wire from the power coming into the floor-mounted dimmer switch to the “out” wire for low beams. While it looks like kicking the switch turns on the high beams in addition to low, that’s not the way the car is actually wired: the low beam is one circuit, and the high beam circuit is a separate one that powers all four lights even though the low-beam circuit is completely off. Since the new circuit we’ve created powers each individually, the jumper wire will still power the low beam circuit when the high beam one (which now powers only those two lights) is turned on. And with that, after several months of wiring Scarlett, we lay aside our crimpers for a welding torch.

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01. We started our lighting upgrade with the Cibie driving lights we installed a couple years ago. While they worked fine, the bulb holder is powered by what appears to be 16-gauge wire, which is relatively small-gauge. We upgraded it to 10-gauge and added a Weatherpack connector so they can be removed and reinstalled more easily.

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02. We upgraded the front turn signals with a switchback LED sourced from Autolumination. With the parking lights on, the switchback LED projects a white light, giving you a little more light on the road and making sure oncoming traffic sees you, but switches to flashing yellow when the turn signals are activated. It’s shown plugged into the turn signal subharness.

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03. We sourced 194 LEDs from Super Bright LEDs (www.superbrightleds.com) to replace the factory corner maker bulbs. Since white LEDs have a tendency to “blow out” the color of the lenses they’re behind, we ordered yellow 194s for the front and red for the back.

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04. Philips makes some of the only legit factory replacement LEDs that meet all applicable standards for automotive use, and we used them for the tail and backup lights, ordering red and white LEDs, respectively.

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05. Although there are units available that replace the entire taillight housing with multiple LEDs, using the Philips LEDs is as simple as changing any other bulb. The white plastic “wings” around the LED itself make the Philips products much easier to deal with than most LEDs.

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06. Our goal was to change every filament bulb in the car over to LED for reduced power draw (and maintenance), and that included the dash bulbs. White (left) and green (right) are shown here, but you’ll also need blue and red for the factory color scheme, or others if your creativity needs to assert itself.

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07. To start on the headlights, you’ll need to lock them in the upward position and remove the screws holding the bezel in place. You may find you need to adjust the height so the bezel can come out; at the correct height, there’s not enough room.

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08. Behind the bezel, the factory sealed-beam lights are clamped in place with a chrome trim ring that you’ll need to unscrew to free the light. There are two sets of screws here: the round-headed screws that hold the trim ring, and the flat ones just to the side that hold the mounting cups and adjust headlight aiming. They’ll need to be readjusted later; for now, just be careful not to break the plastic bases.

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09. Here are the retaining cups that hold the lights, along with their plugs. While the cups will need to be modified for the larger lights we’re installing, the factory plugs will be left alone: the new harness will plug into it, but the harness itself was not modified.

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10. We ordered housings, bulbs, and wiring components from Daniel Stern. Among the parts is this plug, which comes in component form and plugs into the female factory headlight plug. We followed the usual practice of matching the color of the wires to the factory harness.

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11. For our high beams, we used these Cibie housings/reflectors with Narva H1 bulbs. A high-temp ceramic plug comes in component form from Daniel Stern, and we assembled it using 12-gauge primary wire from Wire Barn.

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12. Here is the subharness required to run the new lights, including the plug to the factory harness, the plug that goes to the light itself, and the relay plug. While it’s theoretically possible to run the lights with the factory wires (which are 16- and 18-gauge) it’s better to use relays and heavier gauge wire to reduce heat—including the heat from running all that amperage through the headlight switch, which is how the factory setup works.

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13. The Hella housing we used for the low beams. Supposedly developed in concert with BWM, the housing uses H1 bulbs, but the large base requires modifying the headlight buckets. Also note the smaller gauge wire that’s fated to be replaced with 12-gauge.

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14. We used a die grinder to open up the headlight bucket so the larger Hella housing would fit through it, and made a point of making it large enough that the entire housing and bulb assembly can be removed from the front for ease of changing the bulb. If you don’t build a car with maintenance in mind, you’re only half done.

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15. Here is the rear of the factory headlight bucket after modification. While the Cibie housing required no modification to fit, the Hellas obviously did. After all the grinding we taped off the buckets, bead-blasted the underside and painted them semigloss black. The welded plate is a repair where the factory mounting point for the actuator had cracked.

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16. While it was apart, we also blasted the retaining cups and painted them with hi-temp paint. Note how both the bucket and the retaining cup on the right side had to be modified to clear the larger Hella light.

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17. Factory headlight plugs have both a power and ground, but both of these are diverted to power the relay, so you’ll have to find a way to ground each headlight bulb. We mounted this 3/8-inch ground stud to the rear of the headlight bucket assembly on the driver’s side to use as a common ground for all six bulbs (high beam, low beam, driving lights).

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18. We marked each of the three relays with our Dymo label maker and mounted them in the same location on the passenger side where they’ll be easily accessible and closer to our power source. The waterfall of wires on the left will be wrapped in conduit and mounted across the header with the factory harness.

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19. In keeping with our practice of mounting all the other relays and distribution blocks to the passenger side inner fender, we mounted a fused distribution block nearby where it will be accessible when the hood is opened. We used individual 10-gauge wires to power each relay and 8-gauge to bring the power in. We also labeled each fuse position, including with the amperage fuse needed: HI10, LO10, DL10.